Among the objectives of ergonomic chairs, particularly work chairs, is to support the user in a posture that reduces strain and tends to maintain the spine in a “neutral” position, that is, a posture in which the spine is aligned to enable the seated user to work comfortably for prolonged periods of time. Most chairs are configured to support the sitter, at least initially, in a position with a thigh to torso angle (the “seating angle”) of between about 90-110 degrees. Depending in part on the physical condition of the user it may become fatiguing or even painful to maintain this position for a long period of time. It becomes desirable and more comfortable to sit in an increasingly more open seating angle approaching an optimum neutral posture in which the seating angle is of the order of about 130 degrees. If a chair is not adapted to permit the user to increase his seating angle, he may try to compensate, for example by sliding forward on the seat, thus leaving the lower portion of the back unsupported. Over time, this can cause discomfort or even injury. Should the user wish to maintain an upright position, he may perch on the front edge of the seat in order to angle his thighs downward to increase the seating angle. In this position, however, the user gains no benefit from the back support of the chair and because the front edge of the seat typically is not designed to support the full load of a seated person, this position can become uncomfortable over time.
Many chairs are designed to allow for increasing the seating angle by providing a chair back that can move back with the user's torso to support the back, the increased angle between the chair back and seat providing support for a more comfortable seating angle. Such chairs typically have any of a variety of complex devices such as synchronized linkages as may be found in the Aeron Chair from Herman Miller which uses a heavy spring to counterforce the back support. Another type of back reclining device is a reclining seat lift apparatuses such as offered in the Freedom chair from Humanscale, or the Life chair from Knoll, which uses the user's weight to counterbalance a rearward leaning back and enable the user to recline their torso away from their thighs.
Such reclining-back chairs may tend to have deep seats, which make it difficult or impossible for the sitter to lower his thighs; consequently the sitting angle becomes reduced should the user desire to lean forward toward a work surface. Additionally, there is often little or no back support for a user in forwardly leaning positions. The mechanisms in such chairs typically are designed to tilt the sitter away from the work surface making difficult to access the work surface and tools in reclined positions.